1. Field of the Invention
The present invention relates to a phase change memory where phase change material which is changeable in phase between a first phase state and a second phase state is used for storage cells and a manufacturing method thereof.
2. Related Art
A phase change memory in which information can be recorded or erased by applying electric energy such as current to the phase change memory has been known. Material for a recording layer used in the phase change memory causes phase change reversible between a crystal phase and an amorphous or non-crystal phase due to a temperature rising according to application of electric energy. In general, an electric resistance (or specific resistance) of the crystal phase is low, while that of the amorphous phase is high. The phase change memory is a memory which utilizes such a difference in electric resistance (or specific resistance) between the crystal phase and the amorphous phase to record and reproduce information. Accordingly, it is known that two states obtained by the phase change may be an ordered amorphous phase and a disordered amorphous phase instead of the crystal phase and the amorphous phase, if a sufficient resistance change occurs therebetween.
The phase change memory is generally provided with storage cells, each cell having a lower electrode, an electrically conductive portion provided on the lower electrode, a recording layer made from phase change material and provided on the electrically conductive portion, and an upper electrode provided on the recording layer. It is known that the recording layer made from phase change material is smaller in heat conductivity than the lower and upper electrodes or the electrically conductive portion. Therefore, even if current is caused to flow such that temperature rising occurs only in the vicinity of the electrically conductive portion in the storage cell, since heat stays within the recording layer made from phase change material, a region where temperature rising occurs within the recording layer made from phase change material becomes larger than that of the electrically conductive layer. As a result, when size reduction of the storage cell and further capacity increase are intended, there is such a problem that heating must be conducted to an unnecessary portion so that a necessary amount of current can not be reduced and thermal influence between adjacent storage cells can not be neglected. As means for solving the problem, such a technique that two electrically conductive portions with the same shape, each being formed to have a rectangular outer shape and having a square bored face at its central portion on a side of the recording layer, are arranged in parallel via an insulating film and a recording layer is constituted so as to come in contact with portions of the square bored faces of the two electrically conductive portions to reduce a sectional area on the side of the electrically conductive portions to the minimum so that a contact area between the electrically conductive portions and the recording layer is reduced sufficiently has been known (for example, see Non-Patent Literature 1 (E. Varesi et al., “Advance in Phase Change Memory Technology”, Proceedings of EPCOS 2004, paper 16, http://www.epcos.org/E_PCOSO4.htm.)).
As described in the Non-Patent Literature 1, when a plurality of electrically conductive portions are provided to come in contact with a recording layer made from phase change material, unless a positional relationship among the electrically conductive portions is set to be equal among respective storage cells, it is difficult to obtain the same property among the storage cells. There is such a problem that it is difficult to control properties of the storage cells according to size reduction of the storage cell, namely according to advance to a mass storage memory, because the electrically conductive portion is also reduced in size.